In general, an electric motor includes a stator and a rotor that rotates relative to the stator, and a coil is wound around the stator and a permanent magnet is disposed in the rotor. In accordance with the arrangement relationship between a stator and a rotor, electric motors can fall into an inner rotor type in which a rotor is disposed inside a stator and an outer rotor type in which a rotor is disposed outside a stator. Further, in accordance with the arrangement structure of a permanent structure with respect to a rotor, the electric motors can be classified into an SPM (Surface Permanent Magnet) type in which a permanent magnet is attached to the surface of a rotor and an IPM (Interior Permanent Magnet) in which a permanent magnet is impregnated in a rotor.
In an inner rotor type, a shaft connected to a load is fitted in the center (rotational center) of a rotor, but in an outer rotor type, a frame connected to a load is fitted around a rotor. When a shaft or a frame is firmly and closely combined with a rotor in order, structural stability of an electric motor can be ensured, deterioration of operational performance can be prevented, and torque can be accurately transmitted to a load. However, previous methods of combining a rotor with a shaft or a frame by fitting a shaft in a shaft insertion hole formed at the center of a rotor or fitting a cylindrical frame around a rotor, for example, force-fitting, caulking, and using fasteners are relatively inefficient and cause side effects in electric motors such as performance deterioration, so these methods are difficult to apply.
Rotation fluctuation called cogging torque is generated while an electric motor including a stator and a rotor operates. Cogging torque is one of factors that deteriorate the performance of an electric motor, causing noise or vibration. A method of removing harmonics from a frequency waveform by applying a skew structure to a rotor is used to reduce cogging torque. However, applying a skew structure to a rotor necessarily makes an assembly process difficult, so a reform measure for simplifying a skew structure and improving assembly efficiency is strongly required.
The following Patent Documents 1 and 2 have been disclosed in the related art.
Patent Document 1: Korean Patent Application Publication No. 10-1999-0065920 (Published on Aug. 16, 1999)
Patent Document 2: Korean Patent Application Publication No. 10-1999-0065127 (Published on Aug. 5, 1999)